Although situated within the highly active research field of WiFi sensing, this paper introduces a novel method based on digital receive beamforming using samples of the Channel State Information (CSI) that is able to advance the state-of-the-art. Key contributions include: a) the definition of a new set of features based on the digital receive beamforming technique, and, b) the test of the proposed method over two different environments exploiting a unique training set for both environments. The excellent results achieved in two different environments using the same trained system push the boundaries of the capabilities of current systems.

This paper characterizes the angular dispersion and Second-Order Fading Statistics (SOFS) for sub- THz communication channels in several representative indoor and outdoor environments for different sub- THz frequencies ranging from 125 to 300GHz. In particular, Level Crossing Rate (LCR) and Average Fade Duration (AFD) are studied by extracting Angle-of-Arrival (AoA) measurements available in the existing open literature considering Rician fading conditions. Key findings include the characterization of angular spread, angular constriction, the direction of maximum fading, and Rice K-factor for the considered angular measurements. Furthermore, the impact of various channel and physical parameters, such as velocity and direction of receiver motion, on the SOFS is thoroughly investigated. It has been observed that the environments that represent a widespread angular profile leads to an elevated LCR. Moreover, specific to sub- THz bands that include a dominant Line-of-Sight (LoS) path and a limited number of diffused scattered Multipath Components (MPCs), the Rice K-factor directly influences the SOFS. The provided analysis is of high significance in the design of various aspects at the physical layer of the communication protocol stack, such as antenna design (elements spacing), modulation, coding, and error correction schemes, especially when adaptive schemes are considered. A notable state-of-the-art application of such SOFS is in the design of Ultra Reliable Low Latency Communication (URLLC) services in Beyond 5G/6G communications.

This study focuses on the innovative field of quantum synchronization for satellite-based navigation systems including Global Navigation Satellite Systems (GNSSs) and the Non-Terrestrial Network (NTN) component of future 6G networks integrating both communication and navigation services. By combining a four-qubit system with the theoretical approach of the Lindblad master equation, we transcend the inherent limits of standard synchronization techniques. This achievement represents a quantum leap in satellite-based positioning, highlighting the scalability and cost-effectiveness of our technique for smaller satellites. The study demonstrates the possibility of reducing synchronization errors to less than one meter, significantly improving the reliability and precision of satellite-based navigation systems. The results of this study may contribute to the future development of both user-centric localization systems (typically GNSS systems) and network-centric localization systems (typically through the NTN component of 6G networks), leading to better positioning performance, more flexible multi-functional systems with the potential to limit both cost and size of satellites.

This paper describes the integration of Terrestrial and Non-Terrestrial Networks, wherein space-based network entities collaborate with traditional and emerging terrestrial communication frameworks to furnish pervasive, resilient, and three-dimensional wireless connectivity worldwide toward the 6th Generation of communication networks. This integration supports heterogeneous services, such as enhancing coverage, user experience, system capacity, service reliability, and availability, while also providing high-speed connectivity in remote or disaster-affected areas, improving existing 5th Generation technologies. Various Use Cases are detailed, highlighting the pivotal roles that Non-Terrestrial Networks play in distinguishing between urban/suburban and rural environments, with particular emphasis on transportation ecosystems. Through this analysis, Key Performance Indicators and requirements are delineated to characterize the requisite service quality for these diverse Use Cases. The paper further presents an overview of potential and standards-compliant integrated Terrestrial/Non-Terrestrial architectures, delineating their roles both in backhauling and access across different layers of Non-Terrestrial systems and elements. These insights are derived from studies conducted within the Integrated Terrestrial And Non-Terrestrial Networks (ITA-NTN) project, part of the European Union initiative defined as the Italian National Recovery and Resilience Plan (NRRP) RESTART Research Program.

This paper addresses the crucial challenge of geo-localization of ultra-low power Internet-of-Remote-Things (IoRT) devices placed in remote areas communicating directly with a Low Earth Orbit (LEO) satellite constellation. Accurate localization of IoRT devices is critical in various applications such as asset tracking, logistics, and environmental monitoring. Moreover, location information can also vastly support the employment of 3GPP-based standards for communication over satellite links. Location estimation of such ultra-low power and low-cost devices is only feasible on the satellite side. Use of Global Navigation Satellite System (GNSS) receivers at the IoRT terminals is not advised as they consume high power and add substantial computational complexity and cost. This paper proposes a novel geometric framework that enables the localization of static IoRT nodes on the satellite side using a single-antenna-equipped single LEO satellite. The proposed solution is based on the computation of the Round Trip Time (RTT) which can be used together with the prior available geometric parameters to compute the direct satellite-to-node distance and Angle-of-Arrival (AoA) parameters. Imprecision in RTT measurement translates into an error in the node localization. The error can be reduced by taking RTT measurements at multiple instances along the satellite's orbit, while minimum 2 measurements are required. A numerical analysis is conducted to prove the validity and usefulness of the proposed framework. The impact of the error in RTT measurements on the performance of the proposed localization scheme is also thoroughly investigated.

Recently, the availability of innovative and affordable COTS (Commercial Off-The-Shelf) technological solutions and the ever-improving results of microelectronics and microsystems technologies have enabled the design of ever smaller yet ever more powerful satellites. The emergence of very capable small satellites heralds an era of new opportunities in the commercial space market. Initially applied only to scientific missions, Earth observation and remote sensing, small satellites are now being deployed to support telecommunications services. This review paper examines the operational features of small satellites that contribute to their success. An overview of recent advances and development trends in the field of small satellites is provided, with a special focus on telecommunication aspects such as the use of higher frequency bands, optical communications, new protocols, and advanced architectures.

Combining the use of Extremely High Frequencies (EHF) and Free Space Optics (FSO) for the feeder links of future High Throughput Satellites (HTS) is recognized as a key element to optimize system efficiency in terms of costs, power consumption and to increase the data rate to unprecedented levels (Terabit/s). This paper provides an overview of the main challenges to be faced to make the exploitation of hybrid EHF/FSO links feasible and cost-effective. In this framework, propagation and HW impairments mitigation techniques will play a key role. Therefore, the paper also presents a vision on how the use of SDN/SDR and NFV, extended down to lower layers of the protocol stack, could provide novel, so far unexplored, opportunities for optimizing the performance of such mitigation techniques.

This paper is motivated by the observation that M2M/IoT traffic is rather heterogeneous. By using traffic data collected in a real scenario, we prove that some M2M devices might generate a signaling traffic more similar to the traffic of a smartphone than to the traffic of a traditional M2M device used for metering applications. This makes difficult the task of a mobile operator to understand the impact of the introduction of M2M/IoT devices into the network. The paper presents a classification of the M2M/IoT heterogeneous world in three classes. The classification has been performed using the C-plane attributes and its effectiveness has been assessed by performing a clustering analysis over the U-plane data, which represent the ground truth. We found a good matching between the results of the classification task carried out by using the C-plane data and the results of the clustering task carried out by exploiting the U-plane data. This means that, by using the C-plane data (simpler with respect to using U-plane data) the network operator can understand with a good accuracy which type of M2M devices are active on the network, and what are the applications that they run and the data traffic that they generate. Therefore, the results may be useful for a proper dimensioning and management of the evolution of an EPC network.

In the last few years, there has been a growing interest in developing sensing systems using RF signals of opportunity, especially exploiting radar-based techniques. Long Term Evolution (LTE) signals are excellent candidates as signals of opportunity thanks to their wide availability and penetration in indoor environments. This is the first work investigating the possibility to use LTE signals for crowd density estimation. The proposed approach is not radar-like but it exploits the correlation between the variations of the received LTE signals (in particular, of the Reference Signal Received Power) and the number of people. An experimental evaluation of the performance is carried out in a indoor environment testing three different positions of the LTE receiver. Achieved results in terms of classification accuracy are very promising.

In the last few years, there has been a growing interest on the usage of passive WiFi signals for detecting and classifying human behavior activities. In most generality, the idea underlying most state of the art approaches is to monitor changes in the Channel State Information (CSI) or equivalent spectral metrics, caused by the different multi- path conditions (and their temporal variations) induced by human activity. The (implicit) underlying intuitive assumption is that, in a static environment, i.e. with no moving humans, the multi-path conditions do not vary, and hence the CSI gathered at the passive receiver side should remain substantially stable in time. Goal of this paper is to document, with an extensive set of experimental measurements, a perhaps surprising result: with modern multi-antenna 802.11n/ac access points, such an intuitive assumption does not seem to hold anymore. Rather, the measured CSI time pattern, which differs for different access point brands, appears to be affected by access points' implementation artifacts. Indeed, our measurements unveil frequent fluctuations of the CSI pattern among multiple ``modes". Albeit still preliminary and, at least in part, still requiring a more thorough understanding (that only the reverse engineering of the access points' implementation may ultimately provide), our measurements unveil a phenomenon which may play havoc with several CSI-based human behavior detection approaches proposed in past works, and especially with those based on CSI measured on data frames, opposed to those based on relatively more stable (but still to be reconsidered in sight of our results) Beacon frames.

This paper investigates the possibility to use Channel State Information (CSI) extracted from Long Term Evolution (LTE) signals for signal fingerprinting localization. Being the first work in this direction, several types of signal fingerprinting-based approaches have been compared (e.g., CSI-based vs RSSI-based, statistic vs deterministic matching rule). In particular, the paper proposes a novel CSI-based signal fingerprinting that uses as fingerprint not directly the vector of channel gains per subcarrier, but rather some features extracted from these vectors. This method would greatly reduce the memory requirement of the database as well as the computational complexity of the matching phase. Experimental results, shown for both indoor and outdoor environments, confirm the effectiveness of the proposed method and also provide interesting insights on the use of LTE signal fingerprinting based on CSI.

Radio receivers, besides acting as wireless network nodes participating to the Internet of Things (IoT) communication task, may act as opportunistic sensors participating to the IoT sensing task. In particular, a radio receiver is intrinsically an electronic sensor which may be used for device-free human activity recognition. In this paper, we analyze recent results on how the identification of the human body presence and movement can be carried out analyzing the RF signals transmitted by sources of opportunity. The impact of channel bandwidth, transmission mode, carrier frequency, and signal descriptors on the recognition performance is discussed. Moreover, we present a novel crowd counting system and assess the performance considering two different types of signal descriptors. Results prove the effectiveness of the presented crowd counting system and allow to get more insights into the relation among the specific sensed environment, chosen signal descriptors, and classification accuracy.

This paper presents a WiFi-based device-free crowd counting and occupancy estimation system that can be used in rooms/environments different from the ones in which the training process has been performed. Therefore, crowd counting is achieved without requiring another training phase in each new environment. The proposed approach analyzes the shape of the Doppler spectrum of the received signal which is correlated to the number of people moving in the monitored environment. Unlike a radar-like approach, the use of a reference signal is not required. Experimental results are presented for two different rooms/environments without any constraint on the movements of the volunteers.

This paper focuses on the use of satellite communication systems for the support of Internet of Things (IoT). We refer to the IoT paradigm as the means to collect data from sensors or RFID and to send control messages to actuators. In many application scenarios, sensors and actuators are distributed over a very wide area; in some cases, they are located in remote areas where they are not served by terrestrial access networks and, as a consequence, the use of satellite communication systems becomes of paramount importance for the Internet of Remote Things (IoRT). The enabling factors of IoRT through satellite are: 1) the interoperability between satellite systems and sensors/actuators and 2) the support of IPv6 over satellite. Furthermore, radio resource management algorithms are required to enhance the efficiency of IoT over satellite. In this work, we provide an integrated view of satellite-based IoT, handling this topic as a jigsaw puzzle where the pieces to be assembled are represented by the following topics: MAC protocols for satellite routed sensor networks, efficient IPv6 support, heterogeneous networks interoperability, quality of service (QoS) management, and group-based communications.

Constant-Envelope (CE) multicarrier techniques have been recently proposed in order to enable the use of multicarrier waveforms in channels characterized by severe nonlinear distortions. They are based on a phase modulation applied to the multicarrier signal, generated in the time domain by means of a real-valued DFT. Such techniques, known as CE-OFDM and CE-SC-FDMA, demonstrated their superior performance with respect to conventional OFDMA and SC-FDMA, when applied to nonlinear satellite channels, at the price of reduced spectral efficiency. An open issue still to be evaluated is related to the impact of phase noise on the performance of CE multicarrier systems. This is especially important in view of the use of EHF bands for broadband satellite communications. This paper aims at analyzing the effects of phase noise on CE-OFDM and CE-SC-FDMA, highlighting the different behavior of conventional and CE multicarrier signals in the presence of such impairment. Then, simulation results will be discussed considering two different phase noise masks. The performed analysis demonstrates that CE multicarrier signals have globally better performance when both phase noise and nonlinear distortions are considered. Moreover, the sensitivity to the phase noise is strongly dependent from the modulation order and the angular modulation index.

Several location-aware applications rely on the position estimated by means of Global Navigation Satellite Systems (GNSS), which are known to estimate an accurate position in an open environment. However, the quality of the estimated position is degraded in harsh environments in terms of accuracy and reliability. Liability-critical services, such as location-based charging, transportation, and road tolling, are threatened by the use of an unreliable position of the user, and the level of trust in the estimated position has to be considered to avoid a failure of the full service chain. Such an issue is faced by means of integrity monitoring methods in the field of GNSS. However, when dealing with harsh environments, integrity monitoring techniques designed for aeronautical applications would lead to very conservative results and to the rejection of all the positions obtained. Such conservative approach is based on the theoretical error models for the estimation of the pseudorange standard deviation in open sky. The purpose of this work is to propose an innovative method for estimating the pseudorange standard deviation extrapolating it from measurements of observable data, to assess the confidence level in the obtained positions in relation to the real environment surrounding the user. While measuring the pseudorange standard deviation taking into account environment conditions and receiver accuracy, the user is able to achieve better estimation of the user equivalent range error (UERE). Estimating the UERE from raw pseudorange measurements with the proposed run-time method and its subsequent use in the computation of protection levels using the receiver autonomous integrity monitoring (RAIM) algorithm shows significant improvement in navigation system availability by deriving tight protection levels.

The exploitation of extremely high-frequency (EHF) bands (30-300 GHz) for broadband transmission over satellite links is currently a hot research topic. In particular, the Q-V band (30-50 GHz) and W-band (75-110 GHz) seem to offer very promising perspectives. This paper aims at presenting an overview of the current status of research and technology in EHF satellite communications and taking a look at future perspectives in terms of applications and services. Challenges and open issues are adequately considered together with some viable solutions and future developments. The proposed analysis highlighted the need for a reliable propagation model based on experimental data acquired in orbit. Other critical aspects should be faced at the PHY-layer level in order to manage the tradeoff between power efficiency, spectral efficiency, and robustness against link distortions. As far as networking aspects are concerned, the large bandwidth availability should be converted into increased throughput by means of suitable radio resource management and transport protocols, able to support very high data rates in long-range aerospace scenarios.

In this paper an analysis on the use of Adaptive Coding and Modulation (ACM) techniques for EHF satellite communications is presented. In particular, our analysis is focused on W-band channels and includes the main channel impairments in this frequency band, i.e. rain fading and HPA non-linearity. The aim of the analysis is to identify modifications to the ACM mode switching algorithm to optimize their use at those high frequency bands.

The recently allocated 71-76 GHz and 81-86 GHz bands provide an opportunity for Line Of Sight (LOS) links for directional point-to-point “last mile” links. An efficient use of this spectrum may allow wireless to finally “catch up” with wires, leading to systems such as “multi-Gigabit wireless Ethernet,” and “wireless fiber.” However, the transmission at such a frequency range is characterized by several additional challenges compared to lower frequency bands, from the technological and propagation point of view, which makes difficult to use them efficiently. In this scenario, IR-UWB technology might offer some more degrees of freedom for the design of a highly integrated, low cost transceiver. This work has at its core the design and BER (Bit Error Rate) performance evaluation of an IR-UWB architecture based on an 85 GHz (this frequency belongs to W band/75-110 GHz) up-conversion stage of train of Gaussian pulses having a duration lower than 1 ns. Finally, we compare performance of this architecture with the ones of a more traditional continuous wave communications system with FSK (Frequency Shift Keying) modulation. Simulation results show that BER performance, in presence of RF non-linearities, for an IR-UWB transceiver architecture operating at W band (with same data rate and bandwidth) are better than a coherent BFSK scheme working in a similar scenario.

Within the context of high-data rate transmissions over time varying multipath fading channels, this article presents a new receiver design for orthogonal frequency division multiplexing (OFDM) systems. The received signal is processed jointly in time and frequency domains. A Discrete Wavelet Transform is applied to the received signal before equalization which is based on the minimum mean square error (MMSE) principle. The paper shows that the proposed receiver is able to work also in highly time-variant channels where a traditional frequency domain equalizer gets very bad performance. Moreover, it is shown that this joint time-frequency equalization is able to exploit time diversity without coding. The performance of the proposed equalizer are shown both in case of perfect channel estimation and in presence of a MMSE linear channel estimator.

This letter addresses the application of Linear Dispersion Codes (LDC) in space and frequency domains in Single Carrier-Frequency Domain Equalization (SCFDE) systems. Space-frequency (SF)-LDCs are more suitable than space-time (ST) LDC in high mobility environments. However, the application of LDCs in space and frequency domains in SCFDE systems is not straightforward as in orthogonal frequency division multiplexing (OFDM) systems because in this case there is no direct access to subcarriers at the transmitter. This letter shows that it is possible to induce a target SF symbol dispersion in SCFDE by using a simple linear pre-coder and a conventional LDC that spreads the symbols in time and space domains according to specific ST dispersion matrices. The letter shows how to derive the ST dispersion matrices to be used at the transmitter as a function of the dispersion matrices describing the target SF dispersion. The presented procedure can be applied to both pure transmit diversity and pure multiplexing schemes, which are special cases of LDCs. Therefore, the proposed scheme provides a low complexity implementation of any dispersion in space and frequency over SCFDE systems, which alternatively would require the use of FFT/IFFT also at the transmitter side. The letter also shows a performance comparison of SF-LDCs and ST-LDCs in a typical propagation environment for future radio systems. The comparison confirms the suitability of using the dispersion in frequency rather than time for a wide range of coherence bandwidths.

The evolution of air interfaces for 4G wireless systems often appears as an OFDM-CDMA battle which OFDM is going to win. This paper reviews this point of view and highlights some other features of this evolution. The paper presents a novel multicode DS-CDMA scheme which has the same advantages as OFDMA over frequency selective channels: good performance with low complexity one-tap receiver. Actually, OFDMA can be seen as a special case of the proposed scheme. The analysis of this scheme will help identifying the main features of the evolution of the air interfaces towards 4G.

In this paper, a low complexity transmit diversity scheme for single carrier-frequency domain equalization (SCFDE) is proposed, which is robust in high time-selective channels, (i.e. in high mobility environments), when the channel cannot be assumed constant over two consecutive processed symbols. The proposed scheme is equivalent to space-frequency block code (SFBC) for orthogonal frequency division multiplexing (OFDM) systems. However, the extension of SFBC to SCFDE systems is not straightforward since in SCFDE systems there is no direct access to subcarriers at the transmitter. The proposed scheme outperforms SCFDE-space-time block code (STBC) over fast fading channels also in high frequency selective channels, with a slight decrease in the overall complexity (in terms of complex multiplications).

This paper presents a general framework for applying the Linear Dispersion Codes (LDC) in the space and frequency domains to Single Carrier - Frequency Domain Equalization (SCFDE) systems. Space-Frequency (SF)-LDC are more suitable than Space-Time (ST)-LDC in high mobility environment. However, the application of LDC in space- frequency domain in SCFDE systems is not straightforward as in Orthogonal Frequency Division Multiplexing (OFDM), since there is no direct access to the subcarriers at the transmitter. This paper describes how to build the space-time dispersion matrices to be used at the transmitter as a function of the desired dispersion matrices in the space and frequency domains. The performance comparison of SF-LDC and ST-LDC in a typical propagation environment for future radio systems confirms the suitability of using the dispersion in frequency instead of in time. The sensitivity of SF-LDC with respect to different levels of time and frequency selectivity is also shown.

This paper proposes a novel slow transmit power control (TPC) algorithm which exploits a cross layer approach in order to optimize the energy efficiency of a single link. This optimization process has been applied to a short range wireless link (i.e. IEEE 802.15.3-based transmission) and it takes into account the signal fluctuations due to the multipath fading of typical indoor environments. Analytical results have shown that the proposed TPC can improve the battery life duration of 25% with respect to a non optimized strategy, without losing transmission quality

This paper aims at clarifying the role of the outer loop power control for CDMA satellite systems with on-board power constraints. If the inner loop of the power control is perfect, the channel turns into a AWGN channel and there is no need of the outer loop. In satellite CDMA systems, due to the longer propagation delay with respect to a terrestrial system, the inner loop of power control is only partly able to track power variations due to fast fading. Moreover, the Rice factor, which characterizes the channel statistics, can widely vary even if the user does not move but just because of the change of the elevation angle. Because of that, a wide range of target SNIR (and larger than in typical terrestrial systems) may be necessary to get the same BER performance. Therefore, the outer loop power control turns out to be essential to minimize the dynamic of the power link margins and avoid capacity degradations induced by the systematic use of static link margins. A semi-analytical model for the capacity evaluation has been developed, which is specifically intended for the power-limited satellite-to-mobile link with multi satellite reception. We found that the capacity gain with respect to a pure SNIR-based strategy (i.e., only inner loop) can reach the 40% of the total capacity in a single reception scheme. A smaller, but still noticeable capacity gain of the order of 20 - 30% is observed in presence of satellite diversity. Therefore, any dimensioning of CDMA satellite systems should not neglect this component of the power control.

The issue of the performance degradation of transmission control protocol/Internet Protocol (TCP/IP) over wireless links due to the presence of noncongestion-related packet losses has been addressed with a physical layer approach. The effectiveness of automatic repeat request techniques in enhancing TCP/IP performance depends on the tradeoff between frame transmission delay and residual errors after retransmissions. The paper shows how a truncated power control can be effectively applied to improve that tradeoff so that a higher transmission reliability is provided without increasing the frame transmission delay through the radio link layer and without increasing the energy consumption. An analytical framework has been developed to show the feasibility and effectiveness of the proposed power control. The analytical results, which are carried out assuming a constant multiuser interference level, show that the proposed algorithm turns out to be very effective when the time variability of the channel is high, which-in case of constant interference level-means that the Doppler frequency is high. Simulations carried out in a variable interference scenario show that the proposed strategy is very effective in improving the end-to-end throughput of a TCP connection. Moreover, these improvements are higher when the traffic is more bursty, like in typical Internet data applications such as e-mail and web surfing.

The issue of the performance degradation of TCP protocols over wireless links due to the presence of non-congestion related packet losses has been addressed at the physical layer. The feasibility and effectiveness of a proper power management strategy that allows the performance to be improved without increasing the transmission delay and/or increasing the energy consumption has been analyzed. In particular, the dependence of the key parameters of the proposed solution with respect to the correlation of the fading channel has been evaluated. Furthermore, it is shown that the proposed strategy can be effectively exploited to remove local retransmissions at the link layer.

The paper considers the application of forward error correction (FEC) and automatic repeat request (ARQ) schemes over correlated wireless channels. In particular, the depth of the interleaver and its influence on the frame-error-rate and transmission delay in the presence of correlated fading and delay constraints is investigated. It is found that an increased interleaver depth in a correlated channel, although improving the bit-error-rate, does not always improve the frame-error-rate (FER). Further, it is shown that, in the case of bursty channels, a large interleaver depth is required to improve the FER, thus negatively influencing the delay. In this case, the use of a retransmission scheme can improve delay performance.

Abstract The concept of adaptivity is of paramount importance in the design of future communication systems, in which a careful exploitation of the limited available resources (bandwidth, power, etc.) is required. The potential of channel‐adaptive transmission has been already recognized 30 years ago, but it did not receive much interest at that time. In the last decade, the advent of feasible software radio systems, and hence, the availability of fast flexible and reconfigurable transceivers has renewed interest in adaptive techniques, which include adaptive modulation and coding, adaptive antennas and adaptive equalization techniques. This paper focuses on adaptive modulation and adaptive error control mechanisms. Basic concepts are highlighted and an overview on the achieved results and new trends in this research area are presented. Some results from information theory are also presented, which show the limitations of these techniques and motivate further research on the practical and design issues that have to be addressed to enable performance to reach close to the theoretical limit. Copyright © 2002 John Wiley & Sons, Ltd.

With non-GEO constellations the primary means of counteracting the shadowing and the blockage effects is through the use of the satellite diversity, i.e. multiple satellites in the constellation are simultaneously visible to user. The probability system availability is improved by increasing the probability at least one satellite is in clear line of sight. In the selection diversity (SD) scheme the user is served by one satellite selected on the basis of the better signal to noise ratio. In CDMA systems the spatial diversity provided by the satellite diversity can be more effectively exploited by combining signal replica coming from satellites in view with the aim at increasing the system capacity. Actually, the performance of signal combining/selection diversity schemes depend on the specific propagation scenario and it is not always effective to combine signal replica. In this paper hybrid selection/combining diversity schemes are proposed and a performance comparison in terms of system availability and capacity enhancement is carried out. Furthermore, an analytical methodology of the downlink of DS-CDMA systems which takes into account the power constraints of a satellite environment is described and applied for the capacity assessment of the proposed satellite diversity schemes.